Ion signals are used in a variety of controls for various types of engines in various industries such as light and heavy duty vehicles, locomotives, off-highway equipment, marine vessels and many industrial applications. For example, ion current has been used to detect knock and misfire in lean burn engines, to detect combustion instability in continuous combustion systems, to determine NOx emissions, to control exhaust gas recirculation, etc.
An ion signal varies due to many factors, including A/F (air/fuel) ratio, flame proximity, humidity, and fuel properties. For example, the important fuel properties that affect ion current (and the combustion process) include hydrogen to carbon ratio, distillation range, volatility and cetane number. Variations in the design parameters from one engine to another and in the fuel properties affect the cylinder gas temperature and pressure, mixture formation, and the distribution of the equivalence ratio in the combustion chamber, all of which affect the formation of ions. The ion signal can be thought of as a single equation with many unknowns. While many of the unknowns have a small effect on the ion signal, they can be enough to reduce the effectiveness of the controls.
One example of the ion signal variation is shown in FIGS. 7a and 7b. Under the same conditions, the ion signal (labeled with reference numeral 700) exhibits a smaller or larger “second hump” depending on the fuel type. FIG. 7a shows an incipient knock event where the fuel type is pure natural gas. FIG. 7b shows an incipient knock event where the fuel type is a natural gas and propane mixture with other factors remaining the same. It can be seen that the ion signal variation resulting from a different fuel type is of the same order of magnitude as an incipient knock signal. As a result, the knock detection control isn't as effective with some fuels and could lead to erroneous detections.